In an implantation operation, the surgeon must select the best-adapted prosthesis so that the patient recovers the greatest possible mobility. The prosthesis selection is partly determined by the state of the capsuloligamentary system of the knee. To obtain an optimal force distribution on the prosthesis, said prosthesis must be implanted in perfect alignment with the leg in extension and the joint space, that is, the region separating the femur from the tibia after the bone cuts have been performed, must have a height, corresponding to the shortest distance separating the femur from the tibia, identical in extension and for a 90° flexion of the leg. The equality in the joint space heights ensures an acceptable force distribution in the ligaments during flexion of the leg.
Further, given the knee deterioration, certain ligaments may have retracted with respect to a normal state. The surgeon must thus firstly estimate the behavior of the ligamentary system before implantation of the prosthesis. In particular, the surgeon may have to “relax” certain ligaments before implantation, that is, incise ligaments to enable them to reach a greater length.
To evaluate the height of the joint space and the state of the ligamentary system, the surgeon uses a medical instrument called a distractor which enables spacing the bone portions of the knee joint to measure the height of the joint space while placing the ligamentary system under tension.
FIG. 1 schematically shows an example of a conventional distractor 10. Distractor 10 comprises a base 11 from which two lower arms 11A, 11B intended to rest on the cut end of the tibia extend. Distractor 10 comprises a body 12 capable of sliding with respect to base 11 along a direction substantially perpendicular to the median plane of lower arms 11A, 11B. Body 12 comprises two upper arms 12A, 12B substantially parallel to lower arms 11A, 11B. Each upper arm 12A, 12B is intended to contact a condyle of the femur. Adjustment means 14, comprising an adjustment screw 15, are capable of sliding body 12 with respect to base 11 to adjust the position of upper arms 12A, 12B with respect to lower arms 11A, 11B.
To arrange distractor 10, the knee is maintained in extension and the surgeon performs an incision. The patella is everted, that is, it is displaced laterally with respect to its normal position and inverted, to free enough space to carry out the cutting of the upper end of the tibia and arrange distractor 10. Lower arms 11A, 11B and upper arms 12A, 12B are introduced into the joint space thus freed by the front of the joint, lower arms 11A, 11B being placed in contact with the cut end of the tibia. Body 12 of distractor 10 is displaced with respect to base 11 until upper arms 12A, 12B contact the condyles of the femur. The height of the joint space is then measured.
During the use of distractor 10, a portion of body 12 and of base 11 protrudes out of the joint space by the front of the knee joint. A disadvantage is that it is accordingly necessary to maintain the patella everted for as long as the distractor is used.
A measurement of the joint space and an indirect and qualitative appreciation of the forces exerted on the ligamentary system can thus be obtained only when the knee is in extension. To obtain an estimate of the joint space for another position of the leg, generally 90° of flexion, the surgeon must remove distractor 10 from the joint space, put the patella back in normal position, and finally bend the leg in the desired position. The surgeon relaxes the ligaments if necessary until the distractor, having a thickness which corresponds to the thickness of the joint space measured by the distractor for the leg in extension, is inserted back into the joint space.
Such a distractor thus does not enable following the variation of the joint space and of the forces exerted on the different elements of the joint according to the leg flexion. The selection of the prosthesis is then usually performed empirically from a single direct measurement of the geometry of the joint space. Further, in the case where a relaxing of the ligaments is necessary, said relaxing is performed by only taking into account the forces exerted on the ligamentary system for a given flexion, generally at 90°, which can appear to be unsatisfactory.